5 Month Old Puppy Showing Signs of Aggression

I adopted a Belgian Malinois/Terrier puppy a few months back. She is about 5 months now. We walk her around our neighborhood a few times a day. She does seem to be a bit scared of all the new things around her (people, ducks, other dogs). When she gets scared while walking, she will usually sit. I get right up beside her and we observe what is going on until it has passed or she is comfortable to start walking again. I have two other dogs in the home that she does well with. We have a family dog that comes over every now and then. They do good together. During their first meeting, it took about 30-minutes of controlled introduction to get them comfortable. She was backing away scared and barking during this session, until she finally opened up. I took her to our local park the other day. They have a few different dog parks. I didn’t take her into any of them. Just let her sit and walk on the outside to observe. She didn’t seem to have any issues with this. She has also been in a 6 week puppy training program and has had no issues with other dogs. She always tries to play with the puppies next to her. Zero issue here.

However, today during our afternoon walk, there was a guy with his boxer across the way. When she saw them, she got scared and sat down like she normally does. I stood right beside her and told the guy to cross the street first. As they were crossing, my pup started to back up behind me and into the bushes. I had her on a very tight hold. The guy ended up allowing his dog to get curious. Instead of walking away, he was walking straight at us. The boxer ended up being about 3 feet away trying to say hi. Then my pup lunged with an aggressive bark. Teeth showing and everything. Since I had a tight hold on her, nothing bad happened. But now I’m really concerned. What steps have you taken to correct your puppies aggression. I know I should probably hire a trainer to work with us one on one. But I’ve had a hard time finding trainers that work specifically with Belgians (I’m in Washington State)